1. How do I add Pokemon to the game? How do you give them their official cry?

2.Is it possible to quickly replace an entire tileset in a ROM. For example is there a way to quickly switch out one Pokemon Center design with another?

3. How do you change OW's?

4. Is it possible to make Pokemon walk with you? How would you do that?

5. How do hackers edit the save-file type? The thing that causes some hacks to only work on VBA and not the actual GBA.

A lot of these can be answered by threads already open, so be sure to check next time.
First, always say the ROM you are using.
In Order:
1. Here's a tutorial: Tutorial
2. I'm not sure. Don't work too much with tilesets myself.
3. Another tutorial: Inserting new OWs
4. Research and Development on that, called the Follow Me Script: R+D
5. IDK about that either.